Employment Minister Michaelia Cash says she was unaware the deputy commissioner of the building watchdog had quit over concerns with the former head Nigel Hadgkiss.
Australian Building and Construction Commissioner deputy commissioner Anthony Southall QC tendered his resignation on October 15, but it was not publicly known until Monday when it broke in the media..
Mr Hadgkiss resigned from his role as ABCC boss last month, having admitted to recklessly misrepresenting union rights on ABCC posters and in handbooks.
Media reports on Monday quoted Mr Southall as saying he had no knowledge of the gravity of Mr Hadgkiss' conduct until reading a court judgment, as the pair had not discussed it.
He found it "untenable to continue in the role" and wanted to "totally dissociate himself from the conduct of Mr Hadgkiss".
Senator Cash told a Senate hearing on Monday she had been aware of Mr Southall's resignation since October 15 but did not see a requirement to publicly disclose it.
Asked about the media reports on the reasons for the resignation, she said: "They were not the reasons given in the resignation provided to myself."
The same committee also heard Mr Hadgkiss had decided to resign after receiving a phone call from Australian Public Service Commissioner John Lloyd.
Mr Lloyd said he had made the call after meeting with Senator Cash on September 12.
He said Senator Cash had told him the government was of the view Mr Hadgkiss' position was no longer tenable.
"I later conveyed to Mr Hadgkiss the government's position that he consider tendering his resignation," Mr Lloyd said.
Mr Lloyd said Mr Hadgkiss' initial response was one of "surprise", but after several more phone calls the decision was made to quit.